In order to study the magnetic resonance of nuclei which exhibit widely differing gyromagnetic ratios, it is necessary to provide a circuit for coupling the sample with the transmitter and receiver of the NMR apparatus, and this coupling circuit must be capable of being tuned over a correspondingly wide frequency range. In the case where the RF power dissipated in the sample is relatively high, the high RF voltages and currents experienced in the coupling circuit impose additional performance problems. Problems such as RF voltage breakdown, for example, are exacerbated by the need for compact design which is required by limited radial dimensions imposed by the magnet bore, magnetic field homogeneity characteristics, the need to accommodate a number of accessories and functions in or near the probe body such as apparatus to spin the sample at a desired angle, gradient coils to reduce spatial magnetic inhomogeneities or to impose same for spacial discrimination purposes, sample temperature control means and the like.
A prior art arrangement is known in which a voltage divider is incorporated into the resonance circuit for the purposes of limiting the voltages experienced by the components of a tune and match capacitor network and further incorporating a three pole switching means to select from pairs of capacitances forming an RF voltage divider from among a plurality of such pairs through a 3-pole switching means. This work is described in greater detail in U.S. Pat. No. 4,710,719.